.alpha.,.beta.-Unsaturated carbonyl compounds are generally important intermediates for the manufacture of odorants, vitamins and carotenoids [see, for example, Chem. Ztg. 97, 23-28 (1973) and Chap. VI ("Total Syntheses") in "Carotenoids", Ed. Otto Isler, published by Birkhauser Basel and Stuttgart, 1971]. Their production by acid-catalyzed rearrangement of .alpha.-alkynols has already been described in the nineteen twenties by K. H. Meyer and K. Schuster [Ber. deutsch. Chem. Ges. 55, 819-823 (1922)] and H. Rupe and E. Kambli [Helv. Chim. Acta 9, 672 (1926)]; the isomerization of secondary or tertiary .alpha.alkynols to .alpha.,.beta.-unsaturated carbonyl compounds has also generally become known as the Meyer-Schuster or Rupe-Kambli rearrangement. In the case of the rearrangement of a carbonyl compound having a terminal alkynyl group there are obtained aldehydes, otherwise ketones are the rearrangement products: ##STR1##
wherein R.sup.1 and R.sup.2 each signify hydrogen or an aliphatic or aromatic residue. In addition to citral, the likewise .alpha.,.beta.-unsaturated aldehydes citronellal and hydroxycitronellal are also of particular industrial interest, namely as intermediates for the manufacture of odorants, terpinoids and vitamins; citral itself can be converted, in each case in several process steps, into the important starting materials for the manufacture of d,l-.alpha.-tocopherol (vitamin E) and vitamin A, isophytol or .beta.-ionone [see, for example, "Vitamine I, Fettlosliche Vitamine", Ed. Otto Isler and Georg Brubacher, published by Georg Thieme Stuttgart, New York 1982, the Chapter VI "Total Syntheses" in "Carotenoids" (published by Birkhauser 1971) and the literature references referred to therein].
Depending on the reaction conditions, the rearrangement of dehydrolinalyl acetate catalyzed by silver or copper ions yields, according to G. Saucy et al. [Helv. Chim. Acta 42, 1945-1955 (1959)], a mixture of "allene acetate" (1-acetoxy-3,7-dimethyl-octa-1,2,6-triene) and "diacetate" (1,1-diacetoxy-3,7-dimethyl-octa-2,6-diene), which can hydrolyze to citral: ##STR2##
This rearrangement of dehydrolinalyl acetate is also known as the Saucy-Marbet rearrangement. However, dehydrolinalool can be converted directly into citral using an alkyl, cycloalkyl or aryl orthovanadate or another vanadium catalyst (UK Patent 1,204,754). Disadvantages in the direct conversion are, however, the low yield (about 31-37%) as well as the formation of dark precipitates which lead to the decomposition of the reaction solution. The direct rearrangement of dehydrolinalool is effected substantially more selectively and efficiently using tris(triphenylsilyl)vanadium oxide at about 140.degree. C. [Chimia 27, 383 (1973) as well as Helv. Chim. Acta 59, 1233-1243 (1976)]. In this case yields of about 78% are achieved in paraffin oil as the solvent.
Further publications of the direct rearrangement of dehydrolinalool to citral using vanadium-containing catalysts include the use of polyboroxyvanadoxydiphenylsilane and of polysilylvanadates as the catalysts [Czechoslovakian Patent CS 264, 720/Chem. Abs. 114, 122769a (1991) and, respectively, Mendeleev Commun. 1994, 89]. Whereas in the first process the achieved yield of about 70% is too low commercially, an 80% yield can be achieved with the second process.
A further catalyst for the direct rearrangement of .alpha.-alkynyls, such as, for example, dehydrolinalool, to .alpha.,.beta.-unsaturated carbonyl compounds consists of the combination of a titanium compound, e.g. titanium tetrachloride or tetrabutoxide, with a copper or silver halide [Tetr. Lett. 29, 6253-6256 (1988) and European Patent Publication 0 240 431 A]. However, the use of copper compounds is disadvantageous in this process. Moreover, also in this case, the about 64% yield of citral which is achieved is unsatisfactory.
An interesting variant of the aforementioned Meyer-Schuster rearrangement has been described briefly by C. Y. Lorber and J. A. Osborn in Tetr. Lett. 37, 853-856 (1996); this is the rearrangement of methylbutynol to prenal using a molybdenum catalyst. In this case, methylbutynol is rearranged to prenal in ortho-dichlorobenzene as the solvent in the presence of the catalyst system molybdenyl acetylacetonate, dibutyl sulphoxide and 4-tert.butylbenzoic acid. Although the yield in this rearrangement is indicated to be 97%, the prenal was not isolated from the reaction mixture, but the stated yield was obtained by gas-chromatographical analysis of the crude product. Presumably, it was difficult to work up the reaction mixture in order to isolate prenal.
L. A. Kheifits and co-workers found that dehydrolinalool could be converted into citral only in 28% yield and into 2-hydroxymethyl-1-methyl-3-isopropenylcyclopent-1-ene in 12% yield at 170.degree. C. in a reaction period of 14 hours when a molybdenum catalyst produced from molybdenum oxide and triphenylsilanol was used for the rearrangement [Tetr. Lett. 34, 2981-2984 (1976)].
From the above remarks it is evident that the previously known processes for the catalyzed rearrangement of .alpha.-alkynols, e.g. dehydrolinalool, to .alpha.,.beta.-unsaturated aldehydes, e.g. citral, have serious disadvantages.